As recent models of electrical and electronic parts are becoming smaller in size and thickness, the electrical circuits used in those models are required to have a higher packing density and a finer definition. Interconnections of such miniature circuits are conventionally established by using anisotropic electroconductive adhesives. Among such anisotropic conductive adhesives, those which are shaped in sheet are commonly used in connecting terminals of a circuit board to a flexible circuit. A recent attempt made in the art is to apply an anisotropic conductive adhesive when a semiconductor chip is to be directly mounted on a circuit board (see, for example, JP-A-5-320413 (The term "JP-A" used herein means an unexamined published Japanese patent application)).
In order to establish electrical connections using an anisotropic conductive adhesive, terminals having a plurality of electrodes arranged at spacings of several tens of micrometers to several thousand micrometers and other terminals to be connected that also have similarly spaced electrodes are press bonded to each other, with the anisotropic conductive adhesive being interposed, such that electrical path is formed only between opposed electrodes. However, if the addition of the conductive filler is excessive or not uniform, or if the resin becomes fluid when the opposed terminals are pressed, two adjacent electrodes may occasionally be shorted as shown in FIG. 5.
In order to prevent the shorting of adjacent electrodes, JP-A-62-76215, JP-A-62-176139 and JP-A-63-237372 have proposed the use of a particulate conductive filler having the surfaces of filler particles coated with a thermoplastic resin, and JP-B-8-2995 (The term "JP-B" used herein means an examined Japanese patent publication) has proposed the use of a particulate conductive filler having the surfaces of filler particles coated with a thermosetting resin.
As described above, JP-B-8-2995 discloses a process for producing the particulate conductive filler having the surfaces of filler particles coated with a thermosetting resin. Specifically, a solvent and conductive fine particles (oil phase) that have been already treated with a coupling agent so that reactant A is present on the surface, are dispersed in water (aqueous phase) that has dissolved therein a reactant B which is capable of reacting with reactant A, to thereby form a suspension. By applying heat or by adding a catalyst to the suspension, the two reactants are allowed to react with each other on the surfaces of the conductive filler particles to form an insulating resin.
A problem with this technique is that due to the presence of oil- and water-soluble ingredients, a complicated procedure has been required to isolate and recover the coated microcapsules of conductive filler. As a result, the overall manufacturing process is complicated, involving many steps while lacking the operating efficiency and the adaptability for mass production. If the microencapsulated filler is to be used in fixing IC chips and the like, the entrance of impurity ions such as chloride ions should be avoided by all means since they corrode electrodes and other components to lower the device reliability. To meet this requirement, additional steps of purification may have been necessitated.